This invention relates to an inverter device which generates an AC output having a variable frequency, and more particularly to an inverter device which provides an output containing less high harmonic waves.
To control the speed of rotation of an AC motor a power source of variable output frequency is employed to supply the motor with power of a variable frequency. Commonly used as such a power source is an inverter device.
FIG. 1 is a circuit diagram of a typical three-phase bridge inverter device which has long been used. Transistors are used for switching elements which constitute a three-phase bridge. The arms of the three-phase bridge comprise six transistors, transistors 10.sub.1, 10.sub.2, 10.sub.3, 10.sub.4, 10.sub.5 and 10.sub.6. The point where the transistors 10.sub.1 and 10.sub.2 are connected is connected to a phase output terminal U. Similarly, the point where the transistors 10.sub.3 and 10 .sub.4 are connected is connected to a phase output terminal V, and the point where the transistors 10.sub.5 and 10.sub.6 are connected is connected to a phase output terminal W. A DC power source 12 is connected to the bridge circuit to supply power thereto. A flywheel diode 14.sub.1 is connected between the emitter and collector of the transistor 10.sub.1. A flywheel diode 14.sub.2 is connected between the emitter and collector of the transistor 10.sub.1. Likewise, four other flywheel diodes 14.sub.3, 14.sub.4, 14.sub.5 and 14.sub.6 are connected to the transistors 10.sub.3, 10.sub.4, 10.sub.5 and 10.sub.6, respectively. The flywheel diodes 14.sub.1 to 14.sub.6 are so connected in order to prevent breakdown of the transistors 10.sub.1 to 10.sub.6. Further, a smoothing capacitor 16 is connected between the two terminals of the DC power source 12.
When the transistors 10.sub.1 to 10.sub.6 are turned on and off under PWM (pulse width modulation) control, a three-phase AC output is obtained from the phase output terminals U, V and W. The frequency of the three-phase AC output is changed by varying the switching frequency of the transistors 10.sub.1 to 10.sub.6.
FIGS. 2A to 2E illustrate the waveforms of voltages obtained at principal places of the three-phase bridge inverter device when the transistors 10.sub.1 to 10.sub.6 are turned on and off under PWM control. Of these figures, FIG. 2A shows the waveform of voltage V.sub.U-O obtained between the phase output terminal U and a neutral point O of the DC power source 12 (point O is an maginary point and nonexistent in the circuit of FIG. 1), FIG. 2B the waveform of voltage V.sub.V-O obtained between the phase output terminal V and the neutral point O, and FIG. 2C the waveform of voltage V.sub.W-O obtained between the phase output terminal W and the neutral point O. The voltages waves shown in FIGS. 2A, 2B and 2C are controlled by a control circuit (not shown) so that they become similar and have a phase difference of 120.degree.. FIG. 2D shows the waveform of voltage V.sub.OM-O obtained between the neutral point O and a load neutral point OM (i.e. neutral point of three-phase Y-connection). The voltage V.sub.OM-O is a composite voltage obtained by combining the three-phase output voltages V.sub.U-O, V.sub.V-O and V.sub.W-O. FIG. 2E shows the waveform of voltage V.sub.U-OM obtained between the phase output terminal U and the load neutral point OM. The voltage V.sub.U-OM is a composite voltage obtained by combining the voltage V.sub.U-O and the composite voltage V.sub.OM-O. The voltage V.sub.U-OM will be applied to an AC motor if the inverter device of FIG. 1 is used as the power source for the AC motor. If this is the case, the voltage V.sub.U-OM will be applied through the phase output terminal U. As FIG. 2E shows, the voltage V.sub.U-OM contains many high harmonic waves.
Obviously, it is desired that the output of the inverter device shown in FIG. 1 contain as few high harmonic waves as possible. If an output containing many high harmonic waves is supplied to an AC motor, the operation efficiency of the motor will be reduced and, in addition, torque fluctuation will occur. The inverter device, however, cannot provide an output which contains as few high harmonic waves as desired. It is therefore not suitable as a power source for driving an AC motor.